LTM8083 datasheet(14/24 Pages) AD

14 / 24 page

LTM8083

Rev. 0

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APPLICATIONS INFORMATION

move. This value is determined with the part mounted

to a 95mm × 76mm PCB with four layers.

2. θJCbottom, the thermal resistance from junction to the

bottom of the product case, is determined with all of

the component power dissipation flowing through the

bottom of the package. In the typical µModule reg-

ulator, the bulk of the heat flows out the bottom of

the package, but there is always heat flow out into the

ambient environment. As a result, this thermal resis-

tance value may be useful for comparing packages,

but the test conditions don’t generally match the user’s

application.

3. θJCtop, the thermal resistance from junction to top of

the product case, is determined with nearly all of the

component power dissipation flowing through the top

of the package. As the electrical connections of the typ-

ical µModule regulator are on the bottom of the pack-

age, it is rare for an application to operate such that

most of the heat flows from the junction to the top of

the part. As in the case of θJCbottom, this value may be

useful for comparing packages but the test conditions

don’t generally match the user’s application.

A graphical representation of the aforementioned ther-

mal resistances is given in Figure 6; blue resistances

are contained within the μModule regulator, whereas

green resistances are external to the µModule package.

As a practical matter, it should be clear to the reader that

no individual or sub-group of the three thermal resis-

tance parameters defined by JESD 51-12 or provided

in the Pin Configuration section replicates or conveys

normal operating conditions of a μModule regulator.

For example, in normal board-mounted applications,

never does 100% of the device’s total power loss (heat)

thermally conduct exclusively through the top or exclu-

sively through bottom of the µModule package—as the

standard defines for θJCtop and θJCbottom, respectively.

In practice, power loss is thermally dissipated in both

directions away from the package—granted, in the

absence of a heat sink and airflow, a majority of the

heat flow is into the board.

the bandwidth and phase margin of the circuit, a feedfor-

ward capacitor (CFF) could be added by connecting from

VOUT to FB pin which is in parallel with R5 in Figure 1.

The LTpowerCAD design tool is available to down-

load online to perform specific control loop optimiza-

tion and analyze the control stability and load transient

performance.

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configuration

section of the data sheet are consistent with those param-

eters defined by JESD 51-12 and are intended for use with

finite element analysis (FEA) software modeling tools that

leverage the outcome of thermal modeling, simulation,

and correlation to hardware evaluation performed on a

µModule package mounted to a hardware test board.

The motivation for providing these thermal coefficients

is found in JESD 51-12 (Guidelines for Reporting and

Using Electronic Package Thermal Information).

Many designers may opt to use laboratory equipment and

a test vehicle such as the demo board to anticipate the

µModule regulator’s thermal performance in their appli-

cation at various electrical and environmental operating

conditions to compliment any FEA activities. Without

FEA software, the thermal resistances reported in the

Pin Configuration section are, in and of themselves, not

relevant to providing guidance of thermal performance;

instead, the derating curves provided in this data sheet

can be used in a manner that yields insight and guid-

ance pertaining to one’s application usage, and can be

adapted to correlate thermal performance to one’s own

application.

The Pin Configuration section gives three thermal coeffi-

cients explicitly defined in JESD 51-12; these coefficients

are quoted or paraphrased below:

1. θJA, the thermal resistance from junction to ambient, is

the natural convection junction-to-ambient air thermal

resistance measured in a one cubic foot sealed enclo-

sure. This environment is sometimes referred to as

“still air” although natural convection causes the air to

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